There has been provided a vehicle wireless communication system configured to perform vehicle control, such as door locking/unlocking and engine start, in accordance with a wireless signal to be transmitted and received between a vehicle control device mounted on the vehicle and a portable machine carried by a user. Communication methods between a vehicle control device and a portable machine are roughly divided into three types, namely, a polling method, a passive entry method, and a keyless entry method. According to the polling method, the vehicle control device transmits response request signals at predetermined cycles regardless of the position of the portable machine. According to the passive entry method, the vehicle control device transmits a response request signal to the portable machine when a user approaches or touches a door knob. According to the keyless entry method, the portable machine transmits a signal to the vehicle control device when a user operates the portable machine.
According to the passive entry method, when a user carrying the portable machine approaches or touches a door knob, the vehicle control device transmits a response request signal to the portable machine from each of a plurality of antennas provided to the vehicle. The portable machine receives this response request signal and replies a response signal including an ID code. The vehicle control device collates ID codes upon receipt of this response signal. If matching is successful, the vehicle control device permits door locking/unlocking or engine start. According to the polling method, when a user carrying the portable machine approaches the vehicle, the portable machine receives a response request signal from the vehicle control device and operation similar to the above case is performed subsequently.
There is committed, however, improper communication of cheating as if the portable machine at a far position were located adjacent to the vehicle using a repeater configured to relay a response request signal from the vehicle control device and a response signal from the portable machine. Such improper communication using a repeater is called relay attack. A malicious third party different from an owner of a vehicle may commit a crime such as a theft by unlocking a vehicle door or starting an engine by means of such relay attack.
According to JP 2012-144905 A, a single antenna provided to a vehicle transmits a plurality of temporally differentiated measurement signals having different signal strength levels for a security measure against relay attack. If RSSI (received signal strength) values of the measurement signals detected by a portable machine have a difference not more than a threshold, the communication is determined as relay attack and door locking/unlocking and the like are not permitted. The time difference in transmission of the plurality of measurement signals is set to be small such that communication can be determined as relay attack even when the portable machine moves.
According to JP 2010-121297 A, a portable machine detects strength of an LF signal transmitted from a vehicle and the portable machine replies an RF signal including the LF signal strength. After the vehicle detects reception strength of the RF signal, it is determined whether or not the communication is relay attack by comparison between the RF signal strength and the LF signal strength. The portable machine also measures a response time period from reception of the LF signal to transmission of the RF signal and replies the RF signal including the response time period. The vehicle measures a reply time period from transmission of the LF signal to reception of the RF signal, and it is determined whether or not the communication is relay attack by comparison between the response time period and the reply time period.
Furthermore, JP 2006-342545 A discloses providing a vehicle with a plurality of transmission antennas at different positions as well as providing a portable machine with a plurality of reception antennas having different axis directions. The portable machine detects reception strength of each signal from the plurality of transmission antennas at the plurality of reception antennas, and determines whether or not the communication is relay attack by comparing reception strength ratios among the transmitted signals.
The onboard machine and the portable machine have more loads if the processing performed by the onboard machine and the portable machine for security against relay attack is more complicated.
Meanwhile, recent investigation has revealed that a repeater has reception sensitivity much lower than that of a portable machine. When a plurality of onboard transmission antennas each transmit a signal as in JP 2006-342545 A, a repeater may relay a signal transmitted from only one of the transmission antennas. In this case, it is impossible to determine whether or not the communication is relay attack.